An explanation for pervasive underpressure in the Raton Basin, Colorado and New Mexico
Phillip Nelson, Lawrence Anna, and Nicholas Gianoutsos
Underpressure in the Raton Basin is investigated by converting drillstem test pressures to hydraulic head and examining trends in hydraulic head values relative to outcrop elevations. Although the basin has been heavily drilled for coalbed gas, other objectives have been lightly explored. As a result, only 100 usable drillstem test pressures are available for examination in the entire basin. These tests show that, with the exception of wells on the western flank of the basin, pressure-depth ratios are less than hydrostatic and corresponding head values are well below surface elevation. These observations hold for rocks of Paleozoic, Mesozoic, and Cenozoic age. Geologic structure, outcrop locations and surface topography must all be considered to understand the cause of basin-wide underpressure in this Laramide basin. The western margin of the basin is bounded by the Sangre de Cristo Mountains and is characterized by steeply dipping strata associated with Laramide folding and faulting. The Sierra Grande arch and Apishapa uplift form the eastern and northeastern edges of the basin, respectively. Paleozoic strata are truncated on these uplifts and gently dipping strata of Early Cretaceous age are exposed on the arches. Windows of Jurassic, Triassic, and Permian rocks are also exposed on the arches. Surface elevation rises from approximately 5,000 ft in the east to 8,000 ft in the west. Comparison of hydraulic head values with outcrop elevations indicate that these exposures constitute the pressure equilibration locus for the confined aquifers. The sparse data set is insufficient to resolve all questions, but the available data are consistent with a series of confined aquifers with discharge on the eastern flank of the basin and limited recharge from the surface and the western flank. Discharge exceeds recharge, resulting in hydraulic head that climbs slowly to the west while surface elevation climbs rapidly. The resulting separation between hydraulic head and surface elevation results in underpressure at most locations in the basin.
AAPG Search and Discovery Article #90156©2012 AAPG Rocky Mountain Section Meeting, Grand Junction, Colorado, 9-12 September 2012